Present invention relates to a method of performing butt welding of base metals by plasma keyhole welding and apparatus to be used in the performance of said method.
Plasma keyhole welding is widely used in welding of thick wall plate materials, however applications of this method in welding of thick wall tube materials involve difficult technical problems, and are not used practically.
The reason is that treatment of the crater formed at the end point of plasma keyhole welding differs between plate materials and tube materials. If the base metals are plates, the welding end part including the crater may be cut off together after welding. In the case of tube materials, however, the weld line circles around the tube, and the welding end point overlaps with the welding start point, so that the crater can not be cut off.
The applicant had studied to butt-weld tube materials by plasma keyhole welding, and already obtained sucessful results.
The applicant disclosed the crater treatment method at the welding end point in the Japanese Published Examined Patent application No. SHO. 52-15379, Published Unexamined Patent application No. SHO. 50-2646, and Published Examined Patent application No. SHO. 52-9532. Also the applicant disclosed method of forming keyhole upon start of welding in the Published Unexamined Patent application No. SHO-51-32445 and Published Unexamined Utility Model application No. SHO-54-152229. In addition, the applicant disclosed the configuration of plasma welding torch with respect to tube base metals in the Published Unexamined Patent application No. SHO-52-120247.
Plasma keyhole welding should be performed with due considerations given to various forces, such as gravity acting on the deposited metal, surface tension, plasma gas pressure and back gas pressure, and the balance among the flow of shielding gas, welding current, welding speed and cooling water temperature.
Yet, in order to solve problem of blowhole being formed in the welding start area, it is essential to complete a keyhole by stopping progress of welding for a specified time after start of plasma arc, and then proceed welding.
If stationary time T.sub.1 after start of plasma arc until the tube base metal starts to rotate is too short, welding proceeds while keyhole formation is insufficient, and a blowhole if left in the start position. To the contrary, if stationary time T.sub.1 is too long, the keyhole grows excessively to cause melt-down bead. Hence, it is important to keep stationary time T.sub.1 at a proper duration. To obtain optimum bead by keeping constant stationary time T.sub.1 it is, however, essential that the plasma gas temperature is maintained at a specified high point. Nevertheless, since the plasma arc is cooled from its circumference by cooling water within the welding torch and is concentrated to attain a high temperature, if the temperature of cooling water fluctuates, the degree of concentrativeness and temperature of the plasma arc vary, so that said stationary time T.sub.1 is not fixed to some constant, which is found to cause fluctuations in the quality of products.
In the aforementioned Japanese Published Unexamined Utility model application No. SHO-54-152229, the applicant disclosed a welding equipment capable of preventing from welding while the concentration of plasma is insufficient due to rise of cooling water temperature, by starting a plasma arc when the pressure of cooling water becomes higher than 10 kg/cm.sup.2.